Replication-competent murine retroviruses (RCRs) can be generated in some cases from packaging cell lines used in preparation of gene therapy products generated for human use. Since the risk associated with RCR in man is unknown, rigorous testing for RCRs is recommended to exclude possible contamination of the product. Previous studies in which normal and moderately immunosuppressed monkeys were inoculated with RCR indicated rapid virus clearance and no pathology (Cornetta et al, 1991); however, a recent study has shown RCR to be associated with development of lymphomas in severely immunosuppressed rhesus monkeys (Donahue et al, 1992). To assess the potential risk of RCRs in man, we undertook to analyze RCR infection and lymphomagenesis using monkeys as a primate model system. Four normal juvenile rhesus monkeys were inoculated with a RCR preparation obtained from Genetic Therapy Inc. Blood samples were obtained every two weeks post-inoculation: PBLs were tested for virus isolation; plasma for antibody detection by Western blot analysis; and PBL DNA for viral sequences by PCR. In addition clinical hematology and serum chemistry was carried out. Results indicated that all four monkeys seroconverted. Infectious RCR was recovered from PBLs at early time points PI (4 - 6 weeks PI); however, no virus was recovered late in the infection ( 53 weeks PI). PCR analysis of PBL DNAs indicated presence of RCR sequences at 69 weeks PI. These results indicated that RCR can infect and replicate in normal rhesus monkeys and that viral sequences persist in the host DNA even in absence of active infection. Current studies are directed towards understanding host and viral factors which may activate the latent RCR sequences and its consequences on the host monkey.